It is known in the field of ethylene epoxidation to introduce inert diluents into the reaction zone in order to obtain optimum reaction conditions and/or to avoid the presence of mixtures which are flammable under specific conditions employed. These diluents are called ballast gases or cycle gas diluents. The effectiveness of a number of gases as ballast components has been claimed. Ballast systems disclosed in the past have included methane, ethane, nitrogen, carbon dioxide, and mixtures of two or more of these.
Hydrocarbon ballast gases have a higher heat capacity and thermal conductivity than nitrogen; they facilitate a higher safe oxygen concentration in the cycle gas and assist in moderating the peak reaction temperature. The goal is a higher yield of ethylene oxide at a fixed productivity, or a higher productivity with the same reactor volume and space velocity. Desirable concentrations of ballast gases can be in the range of 30 to 90 mol % of the total feed gas mixture, depending on the explosive limits of the gas mixture employed. Disadvantages to the use of methane or ethane are hydrocarbon purification costs to eliminate sulfur and higher paraffinic hydrocarbons, loss of hydrocarbons through venting of argon and other materials that build up in the process, lower steam generation due to higher heat losses in cycle gas leaving the reactor, and possible safety concerns associated with vapor clouds as a result of rupture-disk failure or equipment damage. Ethane ballast has the additional disadvantage of requiring too much organic halide inhibitor to moderate the reaction; this can cause severe corrosion in the manufacturing unit and pollution problems.
In U.S. Pat. No. 3,119,837 (Jan. 28, 1964) Shell Oil discloses a process wherein methane is used as the ballast gas and is added to the feed to the reaction zone in an amount sufficient to maintain a methane concentration of at least 15 mol %. A related Shell patent is German DE 1,254,137 (Jul. 8, 1968).
In Belgian Patent 707,567 (Jun. 5, 1968) to Halcon International, similar benefits are claimed for the use of an ethane ballast.
U.K. Patent GB 1,382,099 to Halcon (Jan. 29, 1975) discloses processes for ethylene oxidation using various mixed-gas ballasts containing primarily ethane and carbon dioxide. The concentration of ethane is maintained in the range of 10 to 70 vol %. The concentration of carbon dioxide in the reaction mixture is maintained at a level greater than 10% but not more than 70% by volume.
U.K. Patent GB 1,191,983 to Societa Italiana Resine (May 13, 1970) discloses a process for preparing ethylene oxide by catalytic oxidation of ethylene with oxygen in the vapor phase at high temperature (250.degree. to 320.degree. C.) and 1 to 30 atm. pressure. Substantially pure ethylene and oxygen, separately or premixed without gaseous diluents, are introduced into the reactor. The ethylene constitutes more than 86% by volume of the gases entering the reactor. The oxygen comprises only about 4 to 6% of the feed gas. The examples demonstrate only a binary gas mixture; no inert "ballast" is used. Presumably some reaction products would eventually build up in such a system, and change the initial gas concentrations The operating temperatures of U.K. 1,191,983 are 40.degree. to 60.degree. C. above those which will be demonstrated as possible employing the improved process herein disclosed. The improvement in operating temperature, aside from the additional advantages, will indicate the instant invention is a superior process.
E.P. Application 357,292 (Mar. 7, 1990) to ICI suggests that ethylene ballast could be used under a unique set of conditions in which a nitrogen oxide promoter is required. A process is disclosed in which ethylene oxide is produced by contacting a gas stream comprising ethylene and oxygen with a silver-containing catalyst, which is also contacted with a chlorine-containing reaction modifier and an oxide of nitrogen selected from NO.sub.2, N.sub.2 O.sub.4, NO, and N.sub.2 O.sub.3 by means of the gas stream, which may contain from 35 to 92% by volume of ethylene. In this process, the source of oxygen can be air, oxygen-enriched air, or oxygen from liquid air separation. A distinction between E.P.357,292 and the instant invention is that ICI claims that the nitrate or nitrite forming oxides of nitrogen are necessary to produce certain process improvements; the invention disclosed herein shows that no such oxide of nitrogen is necessary.
To one not skilled in the art, it might seem obvious to use ethylene ballast in ethylene epoxidation. Ethylene has a high heat capacity, which suggests that it could be used with relatively high oxygen concentrations in a fuel-rich system without danger of ignition, provided that the mixture composition is well within the fuel-rich region of the flammability curve. Ethylene should also contribute to increased selectivity. Despite this analysis, there historically have been technical problems which have made ethylene ballasts impractical on a commercial scale. One of the biggest obstacles has been with respect to loss of ethylene by venting of impurities that build up in the cyclic process.
Ethylene epoxidation can be accomplished by means of an air-based or oxygen-based process. In an air-based process, oxidation can be carried out by employing oxygen-nitrogen mixtures, preferably air. In the process, unreacted gases may be recycled to the reactor, but the extent of this recycle is limited by the necessity of removing excess nitrogen, which continuously increases as air is added to the oxidation reactor. When nitrogen is removed, an appreciable portion of the unreacted gases are lost with the nitrogen. In order to limit the loss of ethylene under these conditions, the withdrawn gases are mixed with air and passed through one or more additional oxidation reactors in the presence of the silver catalyst under rather drastic conditions; however, this appreciably increases the manufacturing costs. Another disadvantage is that an unsatisfactory rate of conversion is observed, even though fairly good selectivity can be demonstrated.
In all reactors, as nitrogen is vented, some ethylene is lost through the vent as well. With the reactors employing older processes, including air-based processes, a large amount of nitrogen had to be vented and it was impossible not to lose a large amount of ethylene. Therefore, high concentrations of ethylene in the feed gas mixture were not practically feasible.
In addition, there are major differences in purging between air-based and oxygen-based processes. The air-based process requires a substantial purge stream and a staged reaction-absorption system. With the oxygen-based system, there is a reduction in the amount of inert gases introduced into the closed cycle, resulting in almost complete recycle of the unconverted ethylene. However, carbon dioxide is formed and must be removed on a continuous basis. Process vents are also required to prevent accumulation of argon in the recycle gas. Argon is a major impurity in an oxygen supply derived from cryogenic separation of air components. In spite of this purge, the total vent stream in an oxygen-based process is much smaller than in an air-based unit. The operation of the main reactor in an oxygen-based process can be at much higher ethylene concentration than that possible in an air-based process. The small purge gas flows in an oxygen-based system operated with high-purity oxygen make it more feasible to use fuel-rich ballast systems rather than nitrogen. These diluents facilitate the use of higher oxygen concentrations in the recycle and, therefore, improved selectivity and productivity.
With the more modern reactors or with modifications to the unit gas-venting system, it is now possible to reevaluate what feed gas mixtures could reasonably be used to promote optimal reaction conditions and improve catalyst performance. It is now more practical than in the past to use the cycle gas vents to selectively purge nitrogen and argon through the use of membrane separation, pressure-swing absorption, or a combination of both without losing significant amounts of the most expensive raw material, ethylene.
It would represent a substantial advance in the art if it were possible to use a lower temperature in an ethylene epoxidation process. It would be a distinct commercial advantage if the requirement for methane ballast were reduced. It would represent an advance over the art if no oxides of nitrogen were necessary for such a process to be very effective. With the improved invention described herein, it is now possible to have the aforementioned advantages in a process which provides slightly higher selectivities as well.